Present non-volatile digital memories utilize several technologies, including magnetic media, magnetic bubble, floating gate, optical media, and destructive read-out ferroelectric memories. Each of these techniques has certain disadvantages. Magnetic media memories include magnetic disk, magnetic tape, magnetic core, plated wire, and magnetic bubble memories. The magnetic disk and magnetic tape memories involve mechanical systems to access the data, and are therefore slow, bulky, and sensitive to shock and vibration. Magnetic core and plated wire memories are physically large, slow to read and write, and are not compatible with modern very large scale integration (VLSI) electronics. Magnetic bubble memories have long access times to read and write, and do not allow random access addressing of the stored data. Floating gate memories include devices referred to as EEPROMS (electrically-erasable programmable read-only memories) and FLASH memories. Both these terms refer to integrated circuit memories which are capable of reading and writing binary data. These memories store data by means of electrical charge stored on an insulated MOSFET gate. They are slow to write data, slow to erase data, and are very sensitive to radiation. Optical memory devices utilize mechanical devices to access the data. The read and write times of these devices are slow, they are large and heavy, and they are sensitive to shock and vibration.
Ferroelectric memories are a relatively new technology. Methods to detect the state of a ferroelectric memory capacitor fall into two categories: destructive read-out and non-destructive read-out. Destructive read-out alters the polarization state of the ferroelectric memory capacitor during the read process, so the capacitor must be restored to its original state after the read-out in order to preserve the data. This is the memory technique currently in use for ferroelectric memories. There are two disadvantages to the destructive read-out method. The first is that the repeated reading and writing using coercive voltage levels required by the destructive read-out wears out the ferroelectric material, causing it to lose its retentive properties. The second disadvantage is that information stored in the memory that uses destructive read-out may be lost if the memory is upset by an electrical or radiation transient between the time that the ferroelectric capacitor is read and the time when its original state is restored. In other words, the destructive read-out cycle introduces a time period in which the memory is volatile, that is, retained only in the electronic state of the circuit.
Methods of non-destructive read-out of ferroelectric memories have been proposed. One such method attempts to utilize the hysteresis in the conductivity of the ferroelectric at low voltage levels. Another method employs ferroelectric dielectric material as the gate insulator in a MOSFET-like device. The conduction of the transistor depends on the polarization of the gate insulator, providing a non-destructive read mechanism. Another method which might be considered non-destructive or partially non-destructive is to perform the memory read with a partial switching of the ferroelectric. The switching of the ferroelectric is performed until the difference in polarities can be differentiated, and then the switching cycle is reversed. This technique subjects the ferroelectric memory capacitor to only a partial, instead of a full, switching cycle.
These methods for non-destructive read-out of a ferroelectric memory have encountered technical problems concerning the properties of the ferroelectric material which so far have made them commercially impractical.